I have used mostly garbage to power my modding needs, gains were from ~5l/100km to ~3.6l/100km last summer, costs were more or less 0 as attachment bits I would needed anyway as well as paint etc for bumper patching, but to be bit on a safe side let's say a tenner.
If I round up a bit and put 1l/100km for difference it would be 22.51l saved when I drove with full mods, maybe around 30€ savings from that alone, but there are savings from partial mods too, which add bit to that.
If I would driven all the time with full mods, then savings would of been lot more of course, but my car is still at developing stage, doing experiments etc, so I should really while those to development savings.
I hope to get more permanent mods installed relatively soon so that I could make more savings, but as it is of now, there is very small amount of driving that I need to do and it will take long time to do even a fill up, even longer with mods.
I used to get around 1000km with a tank, with mods 1500km, so that much is change in reality, but some of that comes from hypermiling too, so there is no very definitive answer to how much truly because of mods, however savings are more than costs put to mods.
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